Long-lived Spin States Research Articles

Electric Dipole Coupling of a Bilayer Graphene Quantum Dot to a High-Impedance Microwave Resonator.

We implement circuit quantum electrodynamics (cQED) with quantum dots in bilayer graphene, a maturing material platform that can host long-lived spin and valley states. Our device combines a high-impedance (Zr ≈ 1 kΩ) superconducting microwave resonator with a double quantum dot electrostatically defined in a graphene-based van der Waals heterostructure. Electric dipole coupling between the subsystems allows the resonator to sense the electric susceptibility of the double quantum dot from which we reconstruct its charge stability diagram. We achieve sensitive and fast detection of the interdot transition with a signal-to-noise ratio of 3.5 within 1 μs integration time. The charge-photon interaction is quantified in the dispersive and resonant regimes by comparing the resonator response to input-output theory, yielding a coupling strength of g/2π = 49.7 MHz. Our results introduce cQED as a probe for quantum dots in van der Waals materials and indicate a path toward coherent charge-photon coupling with bilayer graphene quantum dots.

Engineering colloidal semiconductor nanocrystals for quantum information processing.

Quantum information processing-which relies on spin defects or single-photon emission-has shown quantum advantage in proof-of-principle experiments including microscopic imaging of electromagnetic fields, strain and temperature in applications ranging from battery research to neuroscience. However, critical gaps remain on the path to wider applications, including a need for improved functionalization, deterministic placement, size homogeneity and greater programmability of multifunctional properties. Colloidal semiconductor nanocrystals can close these gaps in numerous application areas, following years of rapid advances in synthesis and functionalization. In this Review, we specifically focus on three key topics: optical interfaces to long-lived spin states, deterministic placement and delivery for sensing beyond the standard quantum limit, and extensions to multifunctional colloidal quantum circuits.

Hyperpolarized long-lived spin state of methylene protons of 2-bromoethanol obtained from ethylene with non-equilibrium nuclear spin order

In this work we achieve a significant overpopulation (PLLS≈1%) of the long-lived spin state (LLS) of methylene protons in 2-bromoethan(2H)ol (BrEtOD) obtained in a reaction between ethylene with non-equilibrium nuclear spin order and bromine water. Given all protons in ethylene are magnetically equivalent, its nuclear states are classified into nuclear spin isomers (NSIM) with total spin I = 2,1,0. Addition of parahydrogen to acetylene produces ethylene with a population of only those NSIMs with I = 1,0. As a result of the reaction with bromine water the non-equilibrium spin order of ethylene is partly transferred to the singlet LLS involving the two methylene groups of BrEtOD. The 1H NMR signal enhancement (SE≈200) obtained as a result of the LLS readout is approximately equal to the SE of the hyperpolarized BrEtOD obtained with a single π/4 pulse. The LLS relaxation time (TLLS) was shown to be approximately 40 s (≈8T1) in the argon-bubbled sample.

Stark control of solid-state quantum memory with spin-wave storage

Quantum memories for quantum communication need to be able to store photons for an extended time and then to release them on demand. This can be achieved in atomic frequency comb ensemble-based quantum memories by control pulses that transfer the excitation to and from long-lived spin states. However, such pulses can give rise to coherent and incoherent noise due to their interaction with the memory ensemble. In this paper, we experimentally demonstrate the ability to switch off the coherent noise from such control pulses during the echo emission in a spin-wave quantum memory, using the linear Stark effect in rare-earth-ion doped crystals. By applying an electric-field pulse, the echo emission was coherently switched off prior to the first spin transfer pulse, and the stored data pulse was restored only when both an optical recall pulse and a rephasing electrical pulse were applied, giving a high degree of control of both desired and undesired emissions. We estimate the effectiveness of this technique by turning off the free-induction decay of a narrow ensemble of ions. This technique can thus improve the noise performance of spin-wave storage at the single-photon level by quenching coherent optical radiation created by strong control pulses. The method demonstrated here represents a proof of principle that the spin-wave storage scheme can be combined with Stark control. The combined scheme serves as an addition to the toolbox of techniques that can be used to realize a full version of a quantum repeater. Published by the American Physical Society 2024

Multiple Stroboscopic Detection of Long-Lived Nuclear Magnetization for Glutathione Oxidation Kinetics.

Imaging the molecular kinetics of antioxidants by magnetic resonance can contribute to the mechanistic understanding of therapeutic approaches. Magnetic resonance detection of the response to flashes of oxidative stress requires sequential spectroscopy on the same time scale on which reactive oxygen species are generated. To this effect, we propose a single-polarization multiple-detection stroboscopic experiment. We demonstrate this experiment for the follow-up of glutathione oxidation kinetics. On-the-fly stroboscopic detection minimizes the durations necessary for single acquisitions yet necessitates sustaining of magnetization lifetimes. Long-lived proton spin states (LLS) in the cysteine and glycine residues of glutathione with TLLS up to 16 s are reached. Based on 1H LLS, we followed fast oxidation kinetics in the glutathione redox pair GSH/GSSG. This new detection method allows sampling of long-lived spin order multiple times via small flip-angle excitations. This establishes the ground for the follow-up of redox processes detecting GSH/GSSG kinetics as magnetic-resonance biomarker of FLASH oxidative processes on time scales of tens of seconds.

Spin dynamics in quantum dots on liquid helium

Liquid He-4 is free from magnetic defects, making it an ideal substrate for electrons with long-lived spin states. When electrons are localized in quantum dots and a strong magnetic field is applied along the surface, such states may serve as qubit states. A nonuniform magnetic field from a current-carrying wire submerged into helium enables switchable spin-orbit coupling. Bringing the frequency of orbital intradot vibrations close to the Larmor frequency allows for obtaining a strong Rashba electric dipole spin resonance and efficient interdot spin coupling. The authors show that the associated spin relaxation can be made sufficiently slow, permitting high-fidelity gate operations.

A flow electrochemistry-enabled synthesis of 2-substituted N-(methyl-d)piperidines.

A synthesis of N-monodeuteriomethyl-2-substituted piperidines is described. An efficient and readily scalable anodic methoxylation of N-formylpiperidine in an undivided microfluidic electrolysis cell delivers methoxylated piperidine 3, which is a precursor to a N-formyliminium ion and enables C-nucleophiles to be introduced at the 2-position. The isotopically labelled N-deuteriomethyl group is installed using the Eschweiler-Clarke reaction with formic acid-d2 and unlabelled formaldehyde. Monodeuterated N-methyl groups in these molecular systems possess small isotropic proton chemical shift differences important in the investigation of molecules that are able to support long-lived nuclear spin states in solution nuclear magnetic resonance.

Selective Excitation of Long-Lived Nuclear Spin States.

Nuclear magnetization storage, once limited by longitudinal and transverse relaxation lifetimes, T1 and T2, can be prolonged by symmetry-adapted nuclear spin order, i.e. long-lived states (LLS) and long-lived coherences (LLC), which have significantly extended relaxation time constants compared to T1 and T2, respectively. Excitation and/or detection of LLS currently involves pulses covering wide frequency ranges in high-magnetic-field spectrometers. This leads to excitation of unwanted signals that may overlap and interfere with the resonances of interest. Herein, we present a new pulse sequence that converts longitudinal magnetization to LLS and further to detectable magnetization using only frequency-selective pulses. We demonstrate the suitability of this sequence for different J-coupled spin pairs in dipeptide AlaGly and protein Ubiquitin. The newly developed method is adapted for investigations of LLS in complex systems such as proteins and mixtures of metabolites where selected molecular groups are to be investigated separately.

Long-lifetime spin excitations near domain walls in 1T-TaS2

SignificanceThere is an intense ongoing search for two-level quantum systems with long lifetimes for applications in quantum communication and computation. Much research has been focused on studying isolated spins in semiconductors or band insulators. Mott insulators provide an interesting alternative platform but have been far less explored. In this work we use a technique capable of resolving individual spins at atomic length scales, to measure the two-level switching of spin states in 1T-TaS2. We find quasi-1D chains of spin-1/2 electrons embedded in 1T-TaS2 which have exceptionally long lifetimes. The discovery of long-lived spin states in a tractable van der Waal material opens doors to using Mott systems in future quantum information applications.

Investigation of the Stark Effect on a Centrosymmetric Quantum Emitter in Diamond.

Quantum emitters in diamond are leading optically accessible solid-state qubits. Among these, Group IV-vacancy defect centers have attracted great interest as coherent and stable optical interfaces to long-lived spin states. Theory indicates that their inversion symmetry provides first-order insensitivity to stray electric fields, a common limitation for optical coherence in any host material. Here we experimentally quantify this electric field dependence via an external electric field applied to individual tin-vacancy (SnV) centers in diamond. These measurements reveal that the permanent electric dipole moment and polarizability are at least 4 orders of magnitude smaller than for the diamond nitrogen vacancy (NV) centers, representing the first direct measurement of the inversion symmetry protection of a Group IV defect in diamond. Moreover, we show that by modulating the electric-field-induced dipole we can use the SnV as a nanoscale probe of local electric field noise, and we employ this technique to highlight the effect of spectral diffusion on the SnV.

An Examination of Factors Influencing Small Proton Chemical Shift Differences in Nitrogen-Substituted Monodeuterated Methyl Groups

Monodeuterated methyl groups have previously been demonstrated to provide access to long-lived nuclear spin states. This is possible when the CH2D rotamers have sufficiently different populations and the local environment is chiral, which foments a non-negligible isotropic chemical shift difference between the two CH2D protons. In this article, the focus is on the N-CH2D group of N-CH2D-2-methylpiperidine and other suitable CH2D-piperidine derivatives. We used a combined experimental and computational approach to investigate how rotameric symmetry breaking leads to a 1H CH2D chemical shift difference that can subsequently be tuned by a variety of factors such as temperature, acidity and 2-substituted molecular groups.

Centralizer theory for long-lived spin states.

Nuclear long-lived spin states represent spin density operator configurations that are exceptionally well protected against spin relaxation phenomena. Their long-lived character is exploited in a variety of Nuclear Magnetic Resonance (NMR) techniques. Despite the growing importance of long-lived spin states in modern NMR, strategies for their identification have changed little over the last decade. The standard approach heavily relies on a chain of group theoretical arguments. In this paper, we present a more streamlined method for the calculation of such configurations. Instead of focusing on the symmetry properties of the relaxation superoperator, we focus on its corresponding relaxation algebra. This enables us to analyze long-lived spin states with Lie algebraic methods rather than group theoretical arguments. We show that the centralizer of the relaxation algebra forms a basis for the set of long-lived spin states. The characterization of the centralizer, on the other hand, does not rely on any special symmetry arguments, and its calculation is straightforward. We outline a basic algorithm and illustrate advantages by considering long-lived spin states for some spin-1/2 pairs and rapidly rotating methyl groups.

Nuclear Spin Relaxation of Longitudinal and Singlet Order in Liquid-CO2 Solutions.

Hyperpolarization techniques can enormously enhance the NMR signal thus allowing the exploitation of hyperpolarized substrates for in-vivo MRI applications. The short lifetime of hyperpolarized spin order poses significant limitations in such applications. Spin order storage can be prolonged through the use of long-lived spin states. Additionally, the storage of spin polarization–either in the form of longitudinal or singlet order–can be prolonged in low viscosity solutions. Here, we report the use of low viscosity liquid-CO2 solutions to store nuclear spin polarization in the form of longitudinal and singlet order for extended periods. Our results demonstrate that this storage time can be considerably sustained in liquid-CO2 solutions in comparison to other low viscosity solvents, opening up the possibility of new, exciting storage experiments in the future.

Bridging the Gap: From Homogeneous to Heterogeneous Parahydrogen-induced Hyperpolarization and Beyond.

Demonstration of parahydrogen-induced polarization effects in hydrogenations catalyzed by heterogeneous catalysts instead of metal complexes in a homogeneous solution has opened an entirely new dimension for parahydrogen-based research, demonstrating its applicability not only for the production of catalyst-free hyperpolarized liquids and gases and long-lived non-equilibrium spin states for potential biomedical applications, but also for addressing challenges of modern fundamental and industrial catalysis including advanced mechanistic studies of catalytic reactions and operando NMR and MRI of reactors. This essay summarizes the progress achieved in this field by highlighting the research contributed to it by our colleague and friend Kirill V. Kovtunov whose scientific career ended unexpectedly and tragically at the age of 37. His role in this research was certainly crucial, further enhanced by a vast network of his contacts and collaborations at the national and international level.

Magnetization Lifetimes Prediction and Measurements Using Long-Lived Spin States in Endogenous Molecules

Nuclear magnetization storage in biologically-relevant molecules opens new possibilities for the investigation of metabolic pathways, provided the lifetimes of magnetization are sufficiently long. Dissolution-dynamic nuclear polarization-based spin-order enhancement, sustained by long-lived states can measure the ratios between concentrations of endogenous molecules on a cellular pathway. These ratios can be used as meters of enzyme function. Biological states featuring intracellular amino-acid concentrations that are depleted or replenished in the course of in-cell or in-vivo tests of drugs or radiation treatments can be revealed. Progressing from already-established long-lived states, we investigated related spin order in the case of amino acids and other metabolites featuring networks of coupled spins counting up to eight nuclei. We detail a new integrated theoretical approach between quantum chemistry simulations, chemical shifts, J-couplings information from databanks, and spin dynamics calculations to deduce a priori magnetization lifetimes in biomarkers. The lifetimes of long-lived states for several amino acids were also measured experimentally in order to ascertain the approach. Experimental values were in fair agreement with the computed ones and prior data in the literature.

Robust transformation of singlet order into heteronuclear magnetisation over an extended coupling range

Several important NMR procedures involve the conversion of nuclear singlet order into heteronuclear magnetisation, including some experiments involving long-lived spin states and parahydrogen-induced hyperpolarisation. However most existing sequences suffer from a limited range of validity or a lack of robustness against experimental imperfections. We present a new radio-frequency scheme for the transformation of the singlet order of a chemically-equivalent homonuclear spin pair into the magnetisation of a heteronuclear coupling partner. The proposed radio-frequency (RF) scheme is called gS2hM (generalized singlet-to-heteronuclear magnetisation) and has good compensation for common experimental errors such as RF and static field inhomogeneities. The sequence retains its robustness for homonuclear spin pairs in the intermediate coupling regime, characterised by the in-pair coupling being of the same order of magnitude as the difference between the out-of-pair couplings. This is a substantial improvement to the validity range of existing sequences. Analytical solutions for the pulse sequence parameters are provided. Experimental results are shown for two test cases.

In vitro singlet state and zero-quantum encoded magnetic resonance spectroscopy: Illustration with N-acetyl-aspartate.

Magnetic resonance spectroscopy (MRS) allows the analysis of biochemical processes non-invasively and in vivo. Still, its application in clinical diagnostics is rare. Routine MRS is limited to spatial, chemical and temporal resolutions of cubic centimetres, mM and minutes. In fact, the signal of many metabolites is strong enough for detection, but the resonances significantly overlap, exacerbating identification and quantification. Besides, the signals of water and lipids are much stronger and dominate the entire spectrum. To suppress the background and isolate selected signals, usually, relaxation times, J-coupling and chemical shifts are used. Here, we propose methods to isolate the signals of selected molecular groups within endogenous metabolites by using long-lived spin states (LLS). We exemplify the method by preparing the LLSs of coupled protons in the endogenous molecules N-acetyl-L-aspartic acid (NAA). First, we store polarization in long-lived, double spin states, followed by saturation pulses before the spin order is converted back to observable magnetization or double quantum filters to suppress background signals. We show that LLS and zero-quantum coherences can be used to selectively prepare and measure the signals of chosen metabolites or drugs in the presence of water, inhomogeneous field and highly concentrated fatty solutions. The strong suppression of unwanted signals achieved allowed us to measure pH as a function of chemical shift difference.

Spin-phonon relaxation from a universal ab initio density-matrix approach

Designing new quantum materials with long-lived electron spin states urgently requires a general theoretical formalism and computational technique to reliably predict intrinsic spin relaxation times. We present a new, accurate and universal first-principles methodology based on Lindbladian dynamics of density matrices to calculate spin-phonon relaxation time of solids with arbitrary spin mixing and crystal symmetry. This method describes contributions of Elliott-Yafet and D’yakonov-Perel’ mechanisms to spin relaxation for systems with and without inversion symmetry on an equal footing. We show that intrinsic spin and momentum relaxation times both decrease with increasing temperature; however, for the D’yakonov-Perel’ mechanism, spin relaxation time varies inversely with extrinsic scattering time. We predict large anisotropy of spin lifetime in transition metal dichalcogenides. The excellent agreement with experiments for a broad range of materials underscores the predictive capability of our method for properties critical to quantum information science.

Preparation of Long-Lived States in a Multi-Spin System by Using an Optimal Control Method.

The lifetime Ts of a long-lived nuclear spin state (LLS) could be much longer than the longitudinal order T1 . Many spin systems were used to produce long-lived states, including two or more homonuclear spins that couple to each other. For multiple homonuclear spins with rather small chemical shift difference, normally it is difficult to selectively control the spins and then to prepare a LLS. Herein, we present a scheme that prepares different spin orders in a multi-spin system by using optimal control and numerical calculation. By experimentally measuring the lifetime of the states, we find that for a three-spin physical system, although there are many forms of state combinations with different spin orders, each component has its own lifetime.

Unveiling Valley Lifetimes of Free Charge Carriers in Monolayer WSe2.

We report on nanosecond-long, gate-dependent valley lifetimes of free charge carriers in monolayer WSe2, unambiguously identified by the combination of time-resolved Kerr rotation and electrical transport measurements. While the valley polarization increases when tuning the Fermi level into the conduction or valence band, there is a strong decrease of the respective valley lifetime consistent with both electron-phonon and spin-orbit scattering. The longest lifetimes are seen for spin-polarized bound excitons in the band gap region. We explain our findings via two distinct, Fermi-level-dependent scattering channels of optically excited, valley-polarized bright trions either via dark or bound states. By electrostatic gating we demonstrate that the transition-metal dichalcogenide WSe2 can be tuned to be either an ideal host for long-lived localized spin states or allow for nanosecond valley lifetimes of free charge carriers (>10 ns).